Systemic lupus erythematosus (SLE) or lupus is the prototypic autoimmune disease resulting in multiorgan involvement. This anti-self response in SLE patients is characterized by autoantibodies directed against a variety of nuclear and cytoplasmic cellular components. These autoantibodies bind to their respective antigens, forming immune complexes that circulate and eventually deposit in tissues. This immune complex deposition causes chronic inflammation and tissue damage.
Both diagnosing and monitoring disease activity are problematic in patients with SLE. Diagnosis is problematic because the spectrum of disease is broad and ranges from subtle or vague symptoms to life-threatening multi-organ failure. There also are other diseases with multi-system involvement that can be mistaken for SLE, and vice versa. Criteria were developed for the purpose of disease classification in 1971 (Cohen, A S, et al., 1971, Preliminary criteria for the classification of systemic lupus erythematosus. Bull Rheum Dis 21:643-648) and revised in 1982 (Tan, E M, et al., 1982. The 1982 revised criteria for the classification of systemic lupus erythematosus. Arth Rheum 25:1271-1277.) and 1997 (Hochberg, M C. 1997. Updating the American College of Rheumatology revised criteria for the classification of systemic lupus erythematosus. Arth Rheum 40:1725). These criteria are meant to ensure that patients from different geographic locations are comparable. Of the 11 criteria, the presence of four or more, either serially or simultaneously, is sufficient for classification of a patient as having SLE. Although the criteria serve as useful reminders of those features that distinguish lupus from other related autoimmune diseases, they are unavoidably fallible. Determining the presence or absence of the criteria often requires interpretation. If liberal standards are applied for determining the presence or absence of a sign or symptom, one could easily diagnose a patient as having SLE when in fact they do not. Similarly, the range of clinical manifestations in SLE is much greater than that described by the eleven criteria and each manifestation can vary in the level of activity and severity from one patient to another. To further complicate a difficult diagnosis, symptoms of SLE continually evolve over the course of the disease. New symptoms in previously unaffected organs can develop over time. There is no definitive test for SLE and, thus, it is often misdiagnosed.
Monitoring disease activity also is problematic in caring for patients with SLE. Lupus progresses in a series of flares, or periods of acute illness, followed by remissions. The symptoms of a flare, which vary considerably between patients and even within the same patient, include malaise, fever, symmetric joint pain, and photosensitivity (development of rashes after brief sun exposure). Other symptoms of lupus include hair loss, ulcers of mucous membranes and inflammation of the lining of the heart and lungs, which leads to chest pain. Red blood cells, platelets and white blood cells can be targeted in lupus, resulting in anemia and bleeding problems. More seriously, immune complex deposition and chronic inflammation in the blood vessels can lead to kidney involvement and occasionally kidney failure, requiring dialysis or kidney transplantation. Since the blood vessel is a major target of the autoimmune response in lupus, premature strokes and heart disease are not uncommon. Over time, however, these flares can lead to irreversible organ damage. In order to minimize such damage, earlier and more accurate detection of disease flares would not only expedite appropriate treatment, but would reduce the frequency of unnecessary interventions. From an investigative standpoint, the ability to uniformly describe the “extent of inflammation” or activity of disease in individual organ systems or as a general measure is an invaluable research tool. Furthermore, a measure of disease activity can be used as a response variable in a therapeutic trial.
Two of the most commonly used instruments for SLE diagnosis are the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) (Bombardier, C., D. D. Gladman, et al. (1992). Derivation of the SLEDAI. A disease activity index for lupus patients. The Committee on Prognosis Studies in SLE. Arth Rheum 35: 630-40), and the Systemic Lupus Activity Measure (SLAM) (Liang, M. H., S. A. Socher, et al. (1989). Reliability and validity of six systems for the clinical assessment of disease activity in systemic lupus erythematosus. Arth Rheum 32: 1107-18). The SLEDAI includes 24 items, representing nine organ systems. The variables are obtained by history, physical examination and laboratory assessment. Each item is weighted from 1 to 8 based on the significance of the organ involved. For example, mouth ulcers are scored as 2, while seizures are scored as 8. The laboratory parameters that are included in the SLEDAI include white blood cell count, platelet count, urinalysis, serum C3, C4 and anti-dsDNA. The total maximum score is 105. The SLAM includes 32 items representing 11 organ systems. The items are scored not only as present/absent, but graded on a scale of 1 to 3 based on severity. The total possible score for the SLAM is 86. Both the SLEDAI and the SLAM have been shown to be valid, reliable, and sensitive to change over time (Liang, M. H., S. A. Socher, et al. (1989). Reliability and validity of six systems for the clinical assessment of disease activity in systemic lupus erythematosus. Arth Rheum 32:1107-18), and are widely used in research protocols and clinical trials. These indices are particularly useful for examining the value of newly proposed serologic or inflammatory markers of disease activity in SLE.
Despite the obvious utility of these instruments, there are some drawbacks. First, there is not always complete agreement between the SLAM and the SLEDAI in the same set of patients. There are several possible reasons for these discrepancies. Unlike the SLEDAI, the SLAM includes constitutional symptoms such as fatigue and fever, which may or may not be considered attributable to active SLE; this activity index relies on physician interpretation. In addition, the SLEDAI does not capture mild degrees of activity in some organ systems and does not have descriptors for several types of activity, such as hemolytic anemia. For these and other reasons, most studies incorporate more than one measure of disease activity. A general review of the state of the art can be found in Ramsey-Goldman, R. and Manzi, S. Systemic Lupus Erythematosus. In: Goldman and Hatch, Ed. Women and Health. Academic Press, San Diego, Calif. 2000: 704-723.
The complement system consists of a complex network of more than 30 functionally linked proteins that interact in a highly regulated manner to provide many of the effector functions of humoral immunity and inflammation, thereby serving as the major defense mechanism against bacterial and fungal infections. This system of proteins acts against invasion by foreign organisms via three distinct pathways: the classical pathway (in the presence of antibody) or the alternative pathway (in the absence of antibody) and the lectin pathway. Once activated, the proteins within each pathway form a cascade involving sequential self-assembly into multimolecular complexes that perform various functions intended to eradicate the foreign antigens that initiated the response.
The classical pathway is usually triggered by an antibody bound to a foreign particle. It consists of several components that are specific to the classical pathway and designated C1, C4, C2, (in that order in the pathway).
In the classical pathway, the first component C1q is bound to an antigen-antibody complex, activating the pathway. This event is followed by sequential activation of the two serine proteases C1r and C1s. Activated C1s has two substrates, the final two proteins of the classical pathway, namely C4 and C2. Protein C4 is cleaved into C4a and C4b. Protein C2 is cleaved to form C2a and C2b. Fragments C4b and C2a assemble to form C4b2a, which cleaves protein C3 into C3a and C3b, which completes activation of the classical pathway.
Fragments C4b and C3b are subject to further degradation by Factor I. This factor cleaves C4b to generate C4d and also cleaves C3b, to generate iC3b followed by C3d. Thus, activation of the classical pathway of complement can lead to deposition of a number of fragments, including C4d and iC3b on immune complexes or other activating surfaces. These fragments are ligands for complement receptor type 1 (CR1) on erythrocytes or red blood cells.